1. Field of the Invention
This invention relates to the field of photodiode detectors, the field of electromagnetic interference and the field of band-limiting optics. In particular, this invention relates to electromagnetic and optical shielding to reduce background noise from photodiode detectors.
2. Description of the Related Art
A photodiode is a semiconductor device which converts the photon energy of light into an electrical signal by releasing and accelerating current-conducting carriers within the semiconductor. A photodiode behaves like an ordinary signal diode, but is specialized with respect to spectral response and efficiency to optimize internally generated current derived from illumination. In applications, a photodiode is often used as a detector which is optically coupled to a light-emitting-diode (LED) emitter. Examples of such applications include solid-state relays, remote control devices, optical communications and noninvasive biomedical sensors.
A limitation in many photodiode applications is a background noise floor which masks the signal detected by the photodiode. A contributing factor to background noise in a photodiode detector circuit, as in most electronic circuits, is the parasitic coupling of electromagnetic interference (EMI) into the circuit. External sources of EMI vary from power lines and cellular telephones to medical devices such as diathermy, MRI and lasers.
Conventionally, an electromagnetic shield is utilized as an effective method of reducing the effect of EMI-induced noise. Typical shielding techniques involve surrounding potentially affected parts with a "Faraday cage" of conducting material. However, conducting materials are typically opaque to optical signals. Hence, for photodiode applications, prior art electromagnetic shields have typically consisted of optically-transparent conductive materials, such as thin film silver or silver alloy or conductive "screens" having optically transmissive openings. This is illustrated in FIG. 1, which is a cut-away view of a prior art cage 100 containing an optical detector 110. The portions of the cage 100 within the optical path 140 between an emitter 150 and the detector 110 are constructed of a transparent or transmissive conductive material 120. The remainder of the cage 100 is conductive material 130 which may be opaque.
Besides electromagnetic interference, a contributing factor to background noise in photodiode detectors is ambient light. For photodiode applications, prior art ambient light reduction techniques typically consist of placing opaque, polarized or similar light-blocking material externally around the signal optical path and external wavelength filters within the signal optical path. This is illustrated in FIG. 2, which is a cut-away view of a prior art optical enclosure 200 containing an optical detector 110. The portion 220 of the enclosure 200 within the optical path 140 between an emitter 150 and the detector 110 is constructed of a wavelength filtering material. The remainder of the enclosure 200 is light blocking material 230.